The broad objective of this project is to analyze at the molecular level the regulatory mechanisms of altered RNA splicing that are controlled by the formation of pathological MBNL1 mega-complexes in myotonic dystrophy 1 (DM1) patient cells. The genetic defect in DM1 results in the production of mutant RNAs encoding expanded CUG tracts. Abnormally expanded CUG tracts have been shown to form aberrant mega-complexes that contain the alternative splice factor, MBNL1, within the nucleus. Several lines of evidence implicate the formation of these high molecular weight complexes in altered splicing of a subset of physiologically important RNAs and in the subsequent development of DM1 pathology in vivo. To determine the mechanism whereby formation of the MBNL1 mega-complexes alters the splice code in DM1 we propose to purify both normal MBNL1 complexes and the aberrant MBNL1 mega-complexes that develop in DM1 myoblasts. In complementary experiments the role of these complexes in dictating RNA splice site choice will be defined. The Aims of this application are: 1. Purification and functional characterization of normal MBNL1 complexes in spliceosome assembly and RNA catalysis. 2. Purification of MBNL1 mega-complexes from DM1 myoblasts and definition of the mechanics of mega-complex formation in vivo. 3. Elucidation of the mechanisms by which formation of MBNL1 mega-complexes alters the splice code in DM1 myoblasts.